Two bodies were found on Monday morning after a blaze broke out in the early hours at a home in the small town of Bowen Mountain, on the foothills of the NSW Blue Mountains.
Police believe the dead, who are yet to be formally identified, are two unaccounted for children after their father and four siblings escaped the inferno, which caused a roof to collapse.
One body was found on the top floor of the house and the other downstairs.
The deadly fire has triggered an outpouring of support, including from local federal MP Susan Templeman, who thanked firefighters and paid tribute to the family.
"News of two deaths and others taken to hospital after a house fire at Bowen Mountain is deeply distressing," she wrote in a post on social media.
"Sincere condolences to the family and local residents in this close-knit Bowen Mountain community."
With a population of less than 2000 and almost 500 families in the area, according to the 2021 census, Bowen Mountain is known for being a tight-knit, family-friendly community.
Community organisations often put on events like Halloween festivals, Christmas gingerbread house workshops and Easter egg hunts for local children, or Christmas lunches for older residents.
Hawkesbury councillor Mike Creed said the community would wrap its arms around those affected.
"Absolutely devastating news from the house fire at Bowen Mountain," he said on social media.
"My heartfelt condolences are with the family and loved ones of those lost."
Other community members have offered their condolences online, with a fundraiser set up to support the family already raising more than $3500.
NSW Police Superintendent Natalie Roberts said the rest of the family escaped the blaze, which happened on the morning they were all due to move interstate.
The mother of the children - who range in age from three to 16 - had been on her way to their new home in Queensland at the time of the fire, she said.
"Our thoughts and prayers are with the families, the siblings and with the broader community," Supt Roberts said.
Neighbours tried to help with initial efforts to fight the fire but couldn't get inside the property due to the intensity of the blaze, authorities said.
The fire is not being treated as suspicious.
When firefighters arrived, the blaze had burned through much of the home's roof cavity and caused structural damage, Rural Fire Service Superintendent Jonathon Hill said.
About 60 firefighters from Fire and Rescue and the Rural Fire Service were needed to bring the fire under control about 3.30am, but many were forced to withdraw before the roof caved in.
"When firefighters arrived, we had a fully developed fire ... with structural collapse already occurring," Supt Hill said.
"It was deemed too unsafe for firefighters to make entry, to conduct any kind of rescue or search."
Other family members had evacuated from the house by the time emergency services arrived, Fire and Rescue NSW Assistant Commissioner Andrew Faunce said.
Family members told firefighters there were still people inside the burning structure, he said.Â
Mr Faunce urged people to ensure they have a winter fire safety plan and check their smoke alarms are in working order.
Supt Roberts confirmed a father and four children who escaped the blaze were taken to Nepean Hospital for smoke inhalation and minor burns.
Officers have established a crime scene at the home, with an investigation under way into the cause of the fire, NSW Police said earlier.
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